1. Field of the Invention
The present invention relates to an exhaust gas recirculating system for an automobile.
2. Description of the Prior Art
An exhaust gas recirculating system is a system which operates to return part of the exhaust gas emitted from an engine to the intake system (this operation is called recirculation of exhaust gas), thereby reducing the amount of nitrogen oxides in the exhaust gas.
A conventional exhaust gas recirculating system will be briefly described.
FIG. 3 is a view of the whole structure of a conventional exhaust gas recirculating system. Such a system as shown in FIG. 3 is already known.
In FIG. 3, reference number 1 designates the main body of an engine, reference number 2 designates an intake manifold, and reference number 3 designates an exhaust manifold. The black arrows indicate the flow of recirculated exhaust gas, while the white arrows indicate the flow of intake air.
The exhaust manifold 3 and the intake manifold 2 communicate with each other via an exhaust gas recirculating passage 4. An exhaust gas recirculation control valve 5 is arranged in the exhaust gas recirculating passage 4. This exhaust gas recirculation control valve 5 is provided for the purpose of controlling the amount of exhaust gas recirculated.
The exhaust gas recirculation control valve 5 has a valve body 12 which is driven by a diaphragm 11. A valve port 21 provided in the exhaust gas recirculating passage 4 is opened or closed by the valve body 12 whereby the amount of exhaust gas recirculated is controlled.
In the exhaust gas recirculation control valve 5, the inside of a diaphragm case 13 is divided into a first chamber 14 and a second chamber 15 by the diaphragm 11.
The first chamber 14 is opened to the atmosphere. On the other hand, the second chamber 15 is communicated with a negative pressure port 24, formed in a carburetor 25 at a location immediately above a throttle valve 23, via a negative pressure passage 22B.
The diaphragm 11 of the exhaust gas recirculation control valve 5 is driven by negative pressure which is produced in the negative pressure port 24. The exhaust gas recirculation is controlled in the following three ways.
First, when the engine is in an idling condition or when it is in an extremely light-load operating condition, since the throttle valve 23 is in the position indicated by the arrow A, atmospheric pressure is produced in the negative pressure port 24. Therefore, atmospheric pressure prevails in the second chamber 15, and the diaphragm 11 is urged in the direction indicated by the arrow P by the force of a compression coil spring 31, thus causing the valve body 12 to close the valve port 21. As a result, the exhaust gas is not recirculated.
Secondly, when the engine is in a medium-load or a light-load operating condition, since the throttle valve 23 is in the position indicated by the arrow B, negative pressure is produced in the negative pressure port 24. Therefore, negative pressure prevails in the second chamber 15, and the diaphragm 11 is urged in the direction indicated by the arrow Q, thus causing the valve body 12 to open the valve port 21. As a result, the exhaust gas is recirculated.
Thirdly, when the engine is in a high-load operating condition, since the throttle valve 23 is positioned as indicated by the arrow C, atmospheric pressure is produced in the negative pressure port 24. Therefore, atmospheric pressure prevails in the second chamber 15, and the diaphragm 11 is urged in the direction indicated by the arrow P by the force of a compression coil spring 31, thus causing the valve body 12 to close the valve port 21. As a result, the exhaust gas is not recirculated.
As will be understood from the above description, the conventional exhaust gas recirculating system is arranged such as to interrupt exhaust gas recirculation when the engine is in an idling condition, in an extremely light-load operating condition, or in a high-load operating condition. This is for the followings reasons.
Namely, when the engine is in an idling condition, or in an extremely light-load operating condition, the engine is in a region where the amount of mixture supplied to the engine is small. Therefore, if exhaust gas recirculation should be effected at such a time, combustion would become unstable, causing malfunction of the engine.
In addition, the reason for suspending exhaust gas recirculation when the engine is in a high-load operating condition is that the exhausted amount of nitrogen oxides is basically small at such a time, and there is therefore no substantial need to effect recirculation of the exhaust gas.
Particular attention should be paid to the point that no exhaust gas recirculation is effected when the engine is in an idling condition or in an extremely light-load operating condition.
Following the above description of the conventional exhaust gas recirculating system, an explanation will now be given of a traction control system with which automobiles are often equipped, such a system being completely separate from the exhaust gas recirculating system.
A traction control system is an art utilized to prevent wheel slip (i.e. prevent wheels from running without moving the vehicle forward) when the driver rapidly depresses the accelerator pedal in an attempt to suddenly accelerate the automobile.
This type of wheel slip tends to happen when an automobile is being run on a snow-covered road. In the case of a rear wheel drive vehicle, for example, it is the rear wheels that would slip, and when this happens, the driving force cannot be transmitted to the road surface, and the vehicle cannot move forward. A traction control system is provided for preventing this phenomenon of wheel slip.
A conventional traction control system will be briefly described.
FIG. 4 is a view of the whole structure of a conventional traction control system. A system such as that shown in FIG. 4 is already known.
In FIG. 4, reference number 1 designates the main body of an engine, reference number 2C designates an intake manifold, reference number 3C designates an exhaust manifold, reference number 25C designates a carburetor, and reference number 23 designates a throttle valve. Further, reference number 32 designates a second throttle valve, reference number 33 designates a driving device for driving the second throttle valve 32, reference number 34 designates a control device for controlling the driving device 33, and reference number 35 designates a slip sensor. In addition, reference number 36 designates a throttle valve opening detecting sensor for detecting and outputting the opening of the throttle valve 36, and reference number 37 designates a second throttle valve opening detecting sensor for detecting and outputting the opening of the second throttle valve 32. The white arrows indicate the flow of intake air.
The traction control system comprises the second throttle valve 32, the driving device 33, the control device 34, the slip sensor 35, the throttle valve opening detecting sensor 36, and the second throttle valve opening detecting sensor 37.
The operation of the conventional traction control system installed, for instance, in a rear wheel drive vehicle will now be described.
The second throttle valve 32 is fully opened as indicated by the arrow D when the wheels are not experiencing slip. On this occasion, therefore, the situation is the same as the case where the second throttle valve 32 is not provided.
Assume that the wheels begin to slip as a result of the driver rapidly depressing the accelerator pedal in an attempt to suddenly accelerate the vehicle. Since this wheel slip is actually the running of the rear wheels without moving the vehicle forward, when this slip takes place, the rotational speed of the front wheels is lower than that of the rear wheels. Thus, the difference between the rotational speed of the front wheels and that of the rear wheels is detected by the slip sensor 35, and when it is made clear that the front wheels have rotational speed which is lower than that of the rear wheel, the control device 34 judges that slip is taking place. Subsequently, the control device 34 operates to make the driving device 33 close the second throttle valve 32 to a very-small-opening position (indicated by the arrow E). When the second throttle valve 32 is closed to a very small opening position, since the amount of intake air is reduced, the result is the same as when the accelerator pedal is allowed to return. Thus, the output torque of the engine is reduced and the wheel slip ceases. (Since slip occurs when the tires are subjected to torque which is larger than the driving force, no slip will take place if the torque is reduced below the driving force.)
As will be understood from the above description, a conventional traction control system is an art designed to reduce the degree of sensitivity of the accelerator pedal when unnecessarily large torque is produced. In order to cause this drop in the degree of sensitivity of the accelerator pedal, the second throttle valve 32 is provided.
Incidentally, when closing the second throttle valve 32 to a very-small-opening position, it is moved in a two-step manner in which, it is at first rapidly closed to the same opening position as that of the throttle valve 23, and is thereafter closed relatively gradually to a very-small-opening position that indicated by the arrow E).
The reason for this is that a shock might be caused if the second throttle valve 32 were to be rapidly closed to the very-small-opening position (indicated by the arrow E). The throttle valve opening detecting sensor 36 and the second throttle valve opening detecting sensor 37 are provided for making the movement of the throttle valve 32 whereby it is at first closed to the same opening position as that of the throttle valve 23 and then closed relatively gradually to the very-small-opening position (indicated by the arrow E).
For reference, it should be made clear that an alternative art for reducing the degree of sensitivity of the accelerator pedal when unnecessarily large torque is produced by the engine is shown, for example, in Japanese Utility Model Laid-Open No. 59-127847, this being different from a traction control system.
Next, consider a case where an exhaust gas recirculating system is provided in an automobile which is provided with a traction control system as described above.
Such a case is shown in FIG. 5.
FIG. 5 is a view of the whole arrangement of an engine provided with a traction control system and an exhaust gas recirculating system. Such an engine as shown in FIG. 5 was not known prior to the present application.
In FIG. 5, reference number 1 designates the main body of an engine, reference number 2 designates an intake manifold, and reference number 3 designates an exhaust manifold. Reference number 25D designates a carburetor, and reference number 23 a throttle valve. Further, reference number 32 designates a second throttle valve, reference number 33 designates a driving device for driving the second throttle valve 32, reference number 34 designates a control device for controlling the driving device 33, reference number 35 designates a slip sensor, reference number 36 designates a throttle valve opening detecting sensor, and reference number 37 designates a second throttle valve opening detecting sensor.
The exhaust manifold 3 and the intake manifold 2 communicate with each other via an exhaust gas recirculating passage 4. An exhaust gas recirculation control valve 5 is arranged in the exhaust gas recirculating passage 4.
Assume that the rear wheels begin to slip as a result of the driver rapidly depressing the accelerator pedal (not shown) in an attempt to sharply accelerate the automobile.
Because wheel slip begins to take place in the automobile, the traction control system operates to close the second throttle valve 32 to a position as indicated by the arrow E. Because of this operation, the amount of intake air is reduced, thereby substantially bringing the engine into an extremely light-load operating condition, and thus the wheel slip ceases.
On this occasion, since the second throttle valve 32 is closed to a position as indicated by the arrow E, negative pressure prevails in the carburetor downstream of the second throttle valve 32. Therefore, negative pressure is produced in the negative pressure port 24, and accordingly, negative pressure is also produced in the second chamber 15 of the exhaust gas recirculation control valve 5. Because of this, the diaphragm 11 is urged in the direction indicated by the arrow Q, causing the valve body 12 to open the valve port 21. Therefore, the exhaust gas is recirculated in spite of the fact that the engine is substantially brought into an extremely light-load operating condition.
As described above, the combination of the conventional systems shown in FIG. 5 has encountered problems in that exhaust gas is recirculated even when the engine is substantially brought into an extremely light-load operating condition (by the operation of the traction control system).
Consequently, in the prior art, combustion becomes unstable, causing malfunction of the engine.